The present invention relates to a detection for remote release apparatus which monitors a circuit carrying high-voltage between two connecting points A and B and disconnects the circuit from the rest of the high-voltage network by automatically opening two power switches when a short circuit is detected. Circuits of the foregoing type are generally known in the art and are exemplified by the system of FIG. 1. As shown therein, a circuit to be protected, which for instance is in the form of a part of a high voltage line HS between the points designated A and B, is to be disconnected from the rest of the high voltage network by the automatic opening of the two power switches C.sub.1 and C.sub.2 should short circuits occur on the line HS. Such short circuits are frequently caused by lightning striking the line.
In order to solve this problem, the prior art utilized two short circuit detectors D.sub.1 and D.sub.2, preferably developed as distance relays. These distance relays generate a command signal and apply the same to the two switches C.sub.1 and C.sub.2 when a short circuit is detected. The greater the distance between the short circuit and either point A or B, respectively, the greater the time required for D.sub.1 and D.sub.2, respectively, to detect the short circuit. If the location of short circuit is further away from point A or B than about 85% of the length of the line to the detector. It is impossible for one or the other of the detection circuits D.sub.1, D.sub.2 to generate a command signal within a useful period of time. Since, however, at least one of the distance relays will respond to any short circuit located between points A and B, provision can be made to apply the command signal generated by either detection circuit to each of the two switches C.sub.1 and C.sub.2.
In providing a suitable transmission circuit for transmitting the command signal from a local detector (e.g. D.sub.1) to a remote switch (e.g. C.sub.2), the transmission system must be designed with the following considerations in mind:
(A) The release commands occur very infrequently (typically one to ten commands a year). It must therefore be seen to it that no additional false release commands can be produced by disturbances in the transmission systems.
(b) Since a short circuit on the circuit being protected can disturb the signal on the transmission channel, provision must be made to compensate for noise created by a short circuit when a command signal is to be transmitted.